The present invention relates to the general technical field of material treatment, in particular in the field of thin layers, inserts, sheets or strips of material, especially semiconductors, and silicon in particular.
More specifically, the present invention relates to the field of the application of heat treatments to parts by means of light flux.
According to the state of the art, knowledge and methods exist that make use of light fluxes to carry out heat treatments, wherein light fluxes are absorbed on the outer surface and/or in the vicinity of the outer surface of the parts and the heating of the deeper portions takes place by thermal diffusion from the outer surface and/or the vicinity thereof receiving the light flux to the deeper zones of the part to be treated.
These prior art methods are characterised in that the light fluxes used are selected such that the material to be treated is naturally absorbent in terms of the light flux or rendered absorbent by the direct interaction of the light flux with the material of the part, for example using extremely high power levels.
The use of light fluxes proves to be particularly advantageous when it is desired to heat a superficial layer for very short times, in the region of a fraction of a microsecond to some tens of microseconds.
Nevertheless, since the heated thickness generally progresses according to the square root of the time from the outer surface of the part, it is extremely difficult and costly to heat the deep portions of the part and increasingly difficult and costly as the depths increase.
Furthermore, since the thermal profile obtained in the part generally displays a peak on the outer surface and declines in the material from said outer surface, it is necessary to limit the surface temperature to the maximum temperature not to be exceeded by the material within the scope of the method, for example the melting or vaporisation or decomposition point of the material. This limit involves moderate light power fluxes and long periods of time and thus high costs.
Moreover, the document WO 03/075329 proposes heating an absorbent sub-layer with a light flux via a front layer, the heat produced in the sub-layer heating the front layer by diffusion, at a temperature below that reached in the sub-layer.
The present invention relates to a method for heating, at least locally, a wafer comprising at least one layer to be heated and a sub-layer, at least locally, adjacent to said layer to be heated, under the effect of at least one light flux pulse.
The method according to the invention comprises the following steps:
selecting a light flux                wherein the wavelength is such that the absorption coefficient of said flux by the layer to be heated is low as long as the temperature of said layer to be heated is in the low temperature range and said absorption coefficient increases significantly when the temperature of the layer to be heated enters a high temperature range situated approximately above said low temperature range,        and wherein the pulse intensity and duration are such that, in the absence of said sub-layer, the temperature of the layer to be heated remains within said low temperature range;        
selecting a sub-layer                wherein the absorption coefficient of said light flux at said selected wavelength is high in said low temperature range        and wherein the temperature enters the high temperature range when said sub-layer is subject to the light flux;        
and applying said light flux to said wafer, at least locally, via the face of said layer to be heated opposite said sub-layer.
According to the invention, the following mechanism takes place.
In a first phase, the light flux heats the sub-layer from the initial temperature thereof to a temperature at least situated in said high temperature range.
In a second phase, the sub-layer heats, by thermal diffusion, the adjacent portion of the layer to be heated to a temperature situated in said high temperature range.
In a third phase, this adjacent portion thus becoming absorbent and generating, in the layer to be heated, an absorbent thermal front wherein the temperature is situated in said high temperature range, said absorbent to thermal front progresses towards said front face under the combined or dual effect of forward thermal diffusion of the thermal front and a thermal energy supply by said light flux which reaches said thermal front via the not yet absorbent remainder of the layer to be heated.
According to the invention, said low temperature range and said high temperature range can be separated by a behaviour transition threshold of the absorption coefficient as a function of the temperature.
According to the invention, said behaviour transition threshold of the absorption coefficient as a function of the temperature may extend over a temperature range.
According to the invention, the layer to be heated may be low-doped silicon.
According to the invention, the layer to be heated may be a semiconductor material.
According to the invention, said low temperature range may correspond substantially to the range wherein the doping is not intrinsic and said high temperature range may correspond substantially to the range wherein the doping is intrinsic.
According to the invention, the layer to be heated may be gallium nitride.
According to the invention, the sub-layer may be amorphous silicon.
According to the invention, the sub-layer may be high-doped silicon.
According to the invention, the layer to be heated may be gallium nitride and the sub-layer is silicon.
According to the invention, the light flux may be generated by a laser.
FIG. 1 represents a wafer 1 comprising a layer 2 having a front surface 3 and a sub-layer 4 adjacent to the rear surface 5 of the layer 2.
Opposite the front surface 5 of the layer 2, a light flux 7 pulse P generator 6 to said front surface 5 is installed.
In one alternative embodiment, the wafer 1 may comprise a rear layer 8 adjacent to the rear face of the sub-layer 4 to form a substrate.
The layer 2 and the light flux 7 are selected in relation to each other so as to have the following features.
The wavelength of the light flux 7 is such that the absorption coefficient of said flux by the layer 2 is low while the temperature T of said layer 2 is within a low temperature range PBT and the absorption coefficient increases significantly when the temperature T of the layer 2 enters a high temperature range PHT situated approximately above said low temperature range PBT.
The intensity and duration of the pulse supplied by the light flux 7 are such that, in the absence of the sub-layer 4, the temperature of the layer 2 remains within said low temperature range PBT.
The sub-layer 4 and the light flux 7 are selected in relation to each other so as to have the following features.
The absorption coefficient by the sub-layer 4 of the light flux 7 at said wavelength is high in said low temperature range PBT.
The temperature of the sub-layer 4, at least in the thickness thereof adjacent to the layer 2, enters the high temperature range PHT when said sub-layer 4 is subjected to said light flux 7.
With the wafer 1 at an initial temperature situated in the low temperature range PBT, the light flux 7 is applied to the wafer 1 via the front face 3 thereof. The following temperature rises as represented in FIG. 2 then take place.
In the example shown, the upper limit LST of the low temperature range PBT is considered to be less than the lower limit LIT of the upper temperature range PHT. The low temperature range PBT and the high temperature range PHT are separated by a behaviour transition threshold STC of the absorption coefficient as a function of the temperature extending over a transition temperature range.
The mechanism is as follows.
In a first phase, the light flux 7 passes through the layer 2 without the temperature reaching the high temperature range PHT and, reaching the sub-layer 4 in depth, heat said sub-layer 4 from the initial temperature thereof to a temperature at least situated in the high temperature range PHT. The temperature curve 9 is obtained.
In a second phase, the sub-layer 4 forms a transitory heat source and heats, by thermal diffusion, the adjacent portion of the layer 2 to a temperature AT situated in the high temperature range PHT.
In a third phase, this adjacent portion thus becomes absorbent and generates, in depth in the layer 2 to be heated, an absorbent thermal front 10 wherein the temperature FT is situated in the high temperature range PHT, preferably situated above the temperature AT. This absorbent thermal front 10, generated in depth, progresses towards the front face 3 under the combined effect of forward thermal diffusion of the front 10 and a heat energy supply by the light flux 7 which reaches this thermal front 10 via the not yet absorbent remainder of the layer to be heated 2.
When the flux 7 is stopped, the progression of the thermal front 10, according to said propagation mode, stops almost immediately in the layer 2 if it has not reached the front face 3. The progression of the thermal front 10 is thus governed solely by the effects of thermal diffusion.
Under specific constant light flux 7 conditions, the heating of the layer 2 takes place over a thickness substantially in a linear relationship with the duration of the heat flux.
The above sequencing of the above three phases means that they take place in succession over time. However, the description of this succession does not rule out a partial overlap of these phases over time.